1. Field of the Invention
The present invention relates to a device operable for applying thermal energy to a recording medium, the device comprising a thermal head having energisable heating elements which are individually addressable. In particular, the recording medium is a thermographic material, and the head relates to thermal imaging, generally called thermography.
2. Background of the Invention
Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy. Thermography is concerned with materials which are not photosensitive, but are sensitive to heat or thermosensitive and wherein imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material, by a chemical or a physical process which changes the optical density.
Most of the direct thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
In direct thermal printing, the heating of the thermographic recording material may be originating from image signals which are converted to electric pulses and then through a driver circuit selectively transferred to a thermal print head. The thermal print head consists of microscopic heat resistor elements, which convert the electrical energy into heat via the Joule effect. The electric pulses thus converted into thermal signals manifest themselves as heat transferred to the surface of the thermographic material, e.g. paper, wherein the chemical reaction resulting in colour development takes place. This principle is described in xe2x80x9cHandbook of Imaging Materialsxe2x80x9d (edited by Arthur S. Diamondxe2x80x94Diamond Research Corporationxe2x80x94Ventura, Calif., printed by Marcel Dekker, Inc. 270 Madison Avenue, New York, ed. 1991, p. 498-499).
A particular interesting direct thermal imaging element uses an organic silver salt in combination with a reducing agent. An image can be obtained with such a material because under influence of heat the silver salt is developed to metallic silver.
Referring to FIG. 1, there is shown a global principle schema of a thermal printing apparatus 10 that can be used in accordance with the present invention (known from e.g. EP 0 724 964, in the name of Agfa-Gevaert). This apparatus is capable of printing lines of pixels is (or picture elements) on a thermographic recording material m, comprising thermal imaging elements or (shortly) imaging elements, often symbolised by the letters Ie. As an imaging element Ie is part of a thermographic recording material m, both are indicated in the present specification by a common reference number 5. The thermographic recording material m comprises on a support a thermosensitive layer, and generally is in the form of a sheet. The imaging element 5 is mounted on a rotatable platen or drum 6, driven by a drive mechanism (not shown) which continuously advances (see arrow Y representing a so-called slow-scan direction) the drum 6 and the imaging element 5 past a stationary thermal print head 20. This head 20 presses the imaging element 5 against the drum 6 and receives the output of the driver circuits (not shown in FIG. 1 for the sake of greater clarity). The thermal print head 20 normally includes a plurality of heating elements equal in number to the number of pixels in the image data present in a line memory. The image wise heating of the heating element is performed on a line by line basis (along a so-called fast-scan direction X which generally is perpendicular to the slow-scan direction Y), the xe2x80x9clinexe2x80x9d may be horizontal or vertical depending on the configuration of the printer, with the heating resistors geometrically juxtaposed each along another and with gradual construction of the output density.
Each of these resistors is capable of being energised by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element. As the image input data have a higher value, the output energy increases and so the optical density of the hardcopy image 7 on the imaging element 5. On the contrary, lower density image data cause the heating energy to be decreased, giving a lighter picture 7.
In the present invention, the activation of the heating elements is preferably executed pulse wise and preferably by digital electronics. Some steps up to activation of said heating elements are illustrated in FIGS. 1 and 4. First, input image data 16 are applied to a processing unit 18. After processing and parallel to serial conversion (not shown) of the digital image signals, a stream of serial data of bits is shifted (via serial input line 21) into a shift register 25, thus representing the next line of data that is to be printed. Thereafter, under control of a latch enabling line 23, these bits are supplied in parallel to the associated inputs of a latch register 26. Once the bits of data from the shift register 25 are stored in the latch register 26, another line of bits can be sequentially clocked (see ref. nr. 22) into said shift register 25. A strobe signal 24 controls AND-gates 27 and feeds the data from latching register 26 to drivers 28, which are connected to heating elements 29. These drivers 28 (e.g. transistors) are selectively turned on by a control signal in order to let a current flow through their associated heating elements 29.
The recording head 20 is controlled so as to produce in each pixel the density value corresponding with the processed digital image signal value. In this way a thermal hard-copy 7 of the electrical image data is recorded. By varying the heat applied by each heating element to the carrier, a variable density image pixel is formed. A control algorithm must determine for every heating element the amount of energy which must be dissipated. In practice, the controller algorithm must deal with a variety of real-world problems:
Changing characteristics of the film media give different pixel sizes for the same nib (or heating element) energy, e.g. some examples:
a different humidity in the emulsion layer, making its thermal capacity different,
a different chemical composition of the image forming components.
Environmental characteristics like temperature and humidity may change:
a temperature rise of the environment must be taken into account as the image forming temperature will not rise and is given by the chemical composition of the emulsion layer,
humidity again changes the thermal capacity of the emulsion, producing different temperature rises when applying the same amount of energy.
The thermal process itself produces an excessive amount of heat which is not absorbed by the image forming media. This excessive heat is absorbed by a heat sink, but nevertheless, gives rise to temperature gradients internally in the head, giving offset temperatures in the nibs and between the several nibs. E.g. when the image forming process must have an accuracy of 1xc2x0 C. in the image forming media, an increased offset temperature of 5xc2x0 C. in the heat generating element must be taken into account when calculating the power to be applied to that element.
The heat generating elements are in the ideal case fully thermally isolated from each other. In practice, this is never the case and cross-talk exists between the several nibs. This cross-talk can be localised on several levels:
heat transfer between the several nibs in the thermal head structure itself,
heat transfer in the emulsion and film layer itself,
pixels are not printed one aside the other, but partly do overlap on the print media, mechanically mixing heat from one pixel with the other.
The electrical excitation of the nibs is mostly not on an isolated base. This means that not every nib resistor has its own electrical voltage supply which can be driven independent of all the other nibs. In general, some drive signals for driving the nibs are common to each other, this with the purpose of having reduced wiring and drive signals. In general, all nibs can be only switched on or off in the same time-frame. Producing different weighted excitations can only be achieved by dividing the excitation interval in several smaller intervals where for every interval, it can be decided if the individual nib has to be switched on or off. This process of xe2x80x9cslicingxe2x80x9d has its influence on the thermal image forming process. For example, a pattern excitation with the weights (128, 0, 0, 0, 0, 0, 0, 0) differs mathematically only 1 point from a pattern excitation with the weights (0, 64, 32, 16, 8, 4, 2, 1), but the pixelsize often will be much more different than just 1 point. It has been perceived that in some thermal heads, even a xe2x80x98zero-excitationxe2x80x99 or a xe2x80x98no excitationxe2x80x99 interval produces some heat in the nib as well. The controller must take this effect into account.
An empirical way of trying to solve the mentioned problem could comprise the steps of making a printout of all the available slices, measuring the density or pixel size on the pixel output, and deriving a relation between pixel output and the slices used. By simply using a conversion table one could build a continuous monotone and maybe even a linear relation between my table index and the pixel output.
However, such method is not feasible, because of several reasons, among which:
Only the large excitation times (or slices) will give a pixel output.
Also the smaller excitation times are important, as they are still used for compensations, even though no pixel output can be detected on the thermographic material.
Density measurement or pixel size measurement is not always error free, making the evaluation of the results difficult and asking more for a statistical evaluation of the results.
When making a printout, a temperature rise will occur in the printing device, which can jeopardise the whole measurement.
Although it is known to prepare both black-and-white and coloured half-tone images by the use of a thermal printing head, a need for an improved recording method still exists.
It is an object of the present invention to provide an improved method for recording an image on a thermal imaging element by means of a thermal head having energisable heating elements.
Other objects and advantages of the present invention will become clear from the detailed description and the drawings.
The above mentioned object is realised by a method for generating an image on a heat mode imaging element having the specific features defined in the independent claims. Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.